What's causing these artefacts when rendering my baked normal maps?
I'm pretty sure that weird artifacts on your bake were caused by the bad topology of low poly mesh. I 've made a really quick and dirty retopology of the model (it's still, like your previous low poly model, around 50 000 verts). I 've also really quickly unwrapped that mesh and baked the texture.

Here are the results and as you can see there are still some artifacts around UV seams. I'm sure you can do it better than I did if you invest some time in making a good mesh and a UV map.



the normal map bake:





Look in the .BLEND file for more information. I removed the high poly model from it because of the size but you already have it, so I guess that isn't a problem.